The CVs can be categorized into three classes. In the first class, one could include generic CVs such as enthalpy and an entropy surrogate whose aim is to drive crystallization without prior knowledge of the crystal structure.[1]}, [2]} These CVs are useful when exploring polymorphism, but they are not very efficient when it comes to calculating free energies. If the crystal structure is known, a second class of CVs has been designed that describes the local crystalline environment.[3]}, [4]}, [5]}, [6]}, [7]}, [8]} These CVs have been very useful but it is difficult to extend their application to multicomponent systems, and if not properly chosen, they can drive the system towards unwanted defective structures. A third class of CVs, that also involves knowledge of the final crystal structure, is obtained by analyzing the atomic density in the reciprocal Fourier space and using as CVs the Bragg peak intensities that are the experimental signature of crystallinity. Guided by this consideration, we have recently used the peaks in the shperically-averaged structure factor as possible CVs for crystallization.[9]}, [10]}, [11]} When applying this idea in the practice, one usually needs to reduce the number of peaks considered e.g., by linearly combining different peaks with a lengthy trial and error procedure, as done in Refs. zhang2019improving and niu2019temperature.
